AQA A Level Physics复习笔记2.2.5 Quarks & Antiquarks

Quarks

• The three most common flavours of quarks are: up, down and strange
• The majority of hadrons are made up of different combinations of these quarks

The three main quarks that make up most hadrons

• Each quark has a charge, baryon number and strangeness
• These will be provided on the data sheet

Quarks Table provided on the datasheet

• The charge of a hadron is determined by the sum of the charges of its quarks
• For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:

• Equivalently, the baryon number and strangeness of a hadron is determined by the sum of the baryon numbers and strangeness of its quarks

Anti-quarks

• The equivalent antiparticle of the quark is the anti-quark

The three main anti-quarks that make up most hadrons

• These are identical to quarks except with opposite relative charges, baryons numbers and strangeness

Anti-quarks Table

• Quarks have a baryon number of +1/3
• Anti-quarks having a baryon number of –1/3
• Strange quarks have a strangeness of –1
• Anti-strange quarks have a strangeness of +1
  • This is unique to the strange quark

   

Baryons

• Protons and neutrons are not fundamental particles
  • They are each made up of three quarks

   

• Protons are made up of two up quarks and a down quark
• Neutrons are made up of two down quarks and an up quark

Protons and neutrons are made up of three quarks

Mesons

• Pions and kaons are made up of a quark and anti-quark pair
• Pions are either:
  • π+ made up of an up quark and an anti-down quark
  • π– made up on an anti-up quark and a down quark
  • π0 made up of an up quark and anti-up quark or down quark and anti-down quark

   

Pions consist of up and down quarks, and their respective antiparticles, depending on their sign

• Kaons are either:
  • K+ made up of an up quark and an anti-strange quark
  • K– made up on an anti-up quark and a strange quark
  • K0 made up of an down quark and anti-strange quark or anti-down quark and strange quark

   

Kaons consist of up, down and strange quarks, and their respective antiparticles, depending on their sign

Step 1: Calculate the number of protons

The number of protons is from the proton number = 26 protons

Step 2: Calculate the number of neutrons

Number of neutrons = nucleon number – proton number

Number of neutrons = 56 - 26 = 30 neutrons

Step 3: Up quarks in a proton

Protons are made up of uud quarks = 2 up quarks

Step 4: Up quarks in a neutron

Neutrons are made up of udd quarks = 1 up quark

Step 5: Total number of up quarks

26 protons × 2 up quarks = 52 up quarks

30 neutrons × 1 up quark = 30 up quarks

Total number of up quarks = 52 + 30 = 82 up quarks